Systems (wireless power transmission systems) that transmits power in a wireless manner from the ground to movable bodies (such as vehicles) have recently been studied. Patent Document 1 discloses a device in which a power-transmitting coil of a power-transmitting device that is provided on the ground and a power-receiving coil of a power-receiving device that is provided on a vehicle face each other and a battery (secondary battery) is charged, via a charging circuit, with power wirelessly transmitted from the power-transmitting coil to the power-receiving coil. In this device, a DC-DC converter is provided in the power-receiving device such that an input (current, voltage, or power) to the battery is controlled. For example, this DC-DC converter allows the charging of the battery to be controlled by the charging circuit initially based on a constant current (CC) charging type. After some progress of the charging, that is, after the state of charging (SOC: State Of Charge) of the battery has risen to some extent, the DC-DC converter controls the charging type to switch from the CC charging type to a constant voltage (CV) charging type. Thus, the input (current or power) to the battery can be reduced and the battery is charged until reaching a fully-charged state (SOC=100%).